The sphygmomanometric class of automated blood pressure monitors employs an inflatable cuff to exert controlled counter-pressure on the vasculature of a patient. One large class of such monitors, exemplified by that described in U.S. Pat. Nos. 4,349,034 and 4,360,029, both to Maynard Ramsey, III and commonly assigned herewith, employs the oscillometric methodology. In accordance with the Ramsey patents, an inflatable cuff is suitably located on the limb of a patient and is pumped up to a predetermined pressure above the systolic pressure. Then, the cuff pressure is reduced in predetermined decrements, and at each level, pressure fluctuations are monitored. The resultant signals typically consist of a DC voltage with a small superimposed variational component caused by arterial blood pressure pulsations (referred to herein as "oscillation complexes" or just simply "oscillations"). After suitable filtering to reject the DC component and to provide amplification, peak pulse amplitudes (PPA) above a given base-line are measured and stored. As the decrementing continues, the peak amplitudes will normally increase from a lower level to a relative maximum, and thereafter will decrease. These amplitudes thus form an oscillometric blood pressure envelope for the patient. The lowest cuff pressure at which the oscillations have a maximum value has been found to be representative of the mean arterial pressure ("MAP") of the patient. Systolic and diastolic pressures can be derived either as predetermined fractions of MAP, or by more sophisticated methods of direct processing of the oscillation complexes.
The step deflation technique as set forth in the Ramsey patents is the commercial standard of operation. A large percentage of clinically acceptable automated blood pressure monitors utilize the step deflation rationale. When in use in an automatic mode, the blood pressure cuff is placed on the patient and the operator sets a time interval, typically from 1 to 90 minutes, at which blood pressure measurements are to be made. The noninvasive blood pressure ("NIBP") monitor automatically starts a blood pressure determination at the end of the set time interval. Typically, the user selects a short interval if the patient is unstable since unstable blood pressure may change to dangerous levels in a short time and selects a longer interval (typically 5-15 minutes) as the patient becomes more stable. The reason a short interval is not used in all cases is that the probability of trauma to the limb from the cuff inflation increases as the determination frequency increases. Similarly, the frequency of blood pressure determinations may be increased as the patient's blood pressure becomes more unstable.
Unfortunately, setting the interval for blood pressure measurement is not an exact science. If it is wrongly assumed that the patient's blood pressure is stable and a long interval is set, critical minutes may pass before dangerous changes in blood pressure are detected. Conversely, if it is wrongly assumed that the patient's blood pressure is volatile, the patient's limb is subjected to many unnecessary cuff inflations and the possible trauma resulting from such cuff inflations. Since a patient's blood pressure can change rapidly, it is desirable to have an early indication that the patient's physiological condition is changing before the blood pressure falls (or rises) to a dangerous level.
One technique for determining when to initiate an oscillometric blood pressure determination based on changes in the patient's physiological condition is described by Ramsey, III et al. in U.S. Pat. No. 5,606,977, also assigned to the present assignee and hereby incorporated by reference in its entirety. In that system, a so-called "guard mode" is used to perform spot-checks of the patient's blood pressure by inflating the patient's cuff only to diastolic pressure and comparing the oscillometric readings at diastolic pressure to those readings obtained for the last full oscillometric measurement. If the readings have changed substantially, a full oscillometric blood pressure determination is immediately conducted; otherwise, the monitor is idle until the next "guard mode" spot check or the next full oscillometric blood pressure measurement at the expiration of the set duration. This approach allows the duration between full oscillometric blood pressure measurements to be lengthened, thereby reducing the discomfort to the patient.
Currently available multi-parameter patient monitors, such as the DINAMAP.TM. MPS Select Monitor available from Johnson & Johnson Medical, Inc., measure a number of patient parameters besides the patient's blood pressure. For example, the DINAMAP.TM. MPS Select Monitor also measures a patient's heart rate. In the DINAMAP.TM. MPS Select Monitor, the patient's heart rate can be derived from the patient's electrocardiogram (ECG), invasive blood pressure (IP) signal, pulse oximetry (SpO.sub.2) signal, and noninvasive blood pressure (NIBP) signal. The measured heart rate is then presented on the display, and alarms are sounded if the heart rate is outside of selected upper and lower limits.
Useful physiological information about the patient's condition may be obtained from the heart rate. For example, as shown in FIG. 1, heart rate variability (HRV) is a measure in the change in the R-to-R interval (heart period) from beat to beat. HRV can be measured in the time domain, as in FIG. 1, by taking the standard deviation or calculating the coefficient of variation of the heart rate HR(t), or in the frequency domain, as in FIG. 2, by performing a spectral analysis of HR(t) using an FFT, an autoregressive method, or the like, to determine the power at each frequency to yield the power spectral density (PSD(f)) of the heart rate signal. Assessment of the heart rate and HRV provides information about the functional state of the patient's autonomic nervous system, specifically the balance between sympathetic and parasympathetic innervation. Furthermore, reduced HRV and changed blood pressure variability (BPV) are accepted as risk factors in patients with some cardiovascular diseases, metabolic syndromes or neurologic disorders.
As known by those skilled in the art, the body regulates blood pressure by central and peripheral mechanisms. Central mechanisms include changing the inotropic (contractility) and chronotropic (heart rate) state of the heart. Peripheral mechanisms include changing the compliance and/or resistance of large arteries, small arteries and the microvasculature. These mechanisms are controlled in part by neurologic mechanisms. Significant blood loss during surgery will cause a decrease in blood pressure. The body's afore-mentioned control mechanisms will react to try to compensate for the blood loss to maintain a steady blood pressure. One result of these compensatory mechanisms will be an increase in peripheral resistance. The neural changes required to create this increase in peripheral resistance will result in a change in HRV.
Numerous studies have shown that HRV can be used to assess the risk of sudden cardiac death, to evaluate automatic function in diabetics, and to evaluate the depth of anesthesia. Published studies by Turjanmaa et al. in an article entitled "Short-Term Variability of Systolic Blood Pressure and Heart Rate in Normotensive Subjects," Clinical Physiology, Vol. 10, pp. 389-401 (1990), by Baselli et al. in an article entitled "Spectral and Cross-Spectral Analysis of Heart Rate and Arterial Blood Pressure Variability Signals," Computer and Biomedical Research, Vol. 19, pp. 520-534 (1986), and by Akselrod et al. in an article entitled "Hemodynamic Regulation: Investigation By Spectral Analysis," Am. J. Physiol., Vol. 249 (Heart Circ. Physiol. 18), pp. H867-H875 (1985), have also shown that changes in HRV are correlated with changes in blood pressure. While Zapf et al. disclose in U.S. Pat. No. 5,215,096 that the time period for measuring for detecting oscillometric complexes at each blood pressure level in the oscillometric method may be varied according to the heart rate of the patient, the present inventors are not aware that detected changes in HRV have been used to predict changes in blood pressure and to trigger a blood pressure determination.
It is, accordingly, a primary object of the present invention to detect changes in the patient's heart rate variability (HRV) to predict changes in blood pressure and to trigger an NIBP blood pressure determination before the patient's blood pressure drops (or rises) to dangerous levels.
It is a further object of the present invention to automatically determine whether the patient's HRV has changed significantly since the last blood pressure determination so that a new blood pressure determination may be instigated immediately upon detection of a significant change in HRV.
It is also an object of the present invention to provide a technique for monitoring the status of the patient's blood pressure between determinations by monitoring the patient's HRV so that a change in the patient's physiological status between blood pressure determinations will not go undetected.